1. Field of the Invention
This invention relates to a composition for use in the production of integrated circuits, more particularly to a photoresist stripping solution and to a method for its production.
2. Brief Description of the Prior Art
Integrated circuits are produced by coating a semiconductor substrate surface, e.g. a silicon wafer, possibly after a layer of conductive metal such as aluminum has been applied to the surface, with a layer of photoresist material, applying to the photoresist surface a pattern corresponding in either the positive or negative sense to the desired circuit element, activating the printed photoresist layer by exposure to suitable activating radiation, such as light or X-rays, to make the printed and unprinted areas of it differentially soluble in a developing solution and treating the activated photoresist layer with the developing solution to remove the soluble areas of the photoresist layer either to expose the substrate surface, for the application thereto of the desired circuit element which may be by deposition of a conductive metal or metal oxide or by ion implantation with, for example, boron ions into the body of the substrate, or to expose a pre-applied conductive layer which may be etched to remove all but the desired circuit element.
The photoresist layer may be what is known as "positive", that is the activated portions become soluble, or "negative" that is the activated portions become insoluble, on development. A "positive" photoresist may comprise a suitable resin such as a phenol-aldehyde resin, e.g. a phenol-formaldehyde resin, an acrylate or methacrylate resin, a polyvinyl cinnamate resin, or other cross-linkable resin while a "negative" photoresist material may comprise, for example, a polyisoprene material. The photoresist layer is commonly baked at elevated temperature subsequent to development to ensure that cross-linking takes place.
After the application of a desired integrated circuit element to the substrate, the residue of the photoresist layer must be removed to a very high standard of efficiency in preparation for the application of a further integrated circuit element. Photoresist stripping solutions have been developed for this purpose on the basis of phenolic compounds, but such solutions present environmental and disposal problems.
It has been proposed in Japanese Patent Publication No. 50-101107 to strip photoresists by immersing a semiconductor wafer bearing the photoresist in concentrated sulphuric acid to which hydrogen peroxide has been added to cause an exothermic reaction resulting in the decomposition of the hydrogen peroxide with the evolution of nascent oxygen. The wafer is introduced into the hot solution during or immediately after the exothermic reaction the nascent oxygen being postulated to react with the components of the photoresist film so as to degrade it to form carbon dioxide and water. When the temperature of the solution has decreased to 30.degree. C., sulphur trioxide is added to replenish the sulphuric acid and a further quantity of hydrogen peroxide is added, whereupon the solution temperature is raised under the influence of the same exothermic reaction and further nascent oxygen is evolved enabling the stripping process to be resumed. The process may be continued over a number of cycles thereby enabling the extended use of the original stripping solution. The process described in the aforementioned Japanese patent publication would result in undue attack on aluminum coatings present on a wafer.
U.S. Pat. No. 3,900,337 discloses the stripping of photoresists by the use of a mixture of hydrogen peroxide and concentrated sulphuric acid made up by adding the sulphuric acid to the hydrogen peroxide. This causes the temperature of the mixture to increase to 70.degree. C. After subsequent cooling to room temperature the resulting solution may be used. This solution has a lifetime of 3 weeks but only if reagent grade sulphuric acid and hydrogen peroxide are used. The aforementioned U.S. Patent Specification makes a point of the order of mixing of the sulphuric acid and the hydrogen peroxide so as to attain the generation of heat. As a consequence of this a proportion of the hydrogen peroxide will have decomposed to produce nascent oxygen which is lost and it has been found that a stripping solution produced according to this procedure, if shaken, will release a further quantity of oxygen resulting in bubbling and effervescence. The quantity of water present in the sulphuric acid utilised in the above procedure is critical to the effectiveness of the solution for which reason the concentration of the sulphuric acid must be at least 95% wt. and even at that limiting concentration 85% by weight hydrogen peroxide must be used.
The present invention relates to a new or improved process for the production of photoresist stripping solutions. Besides extending the range of materials usable for the production of such compositions, the present invention also enables new and more stable photoresist stripping compositions to be produced and such compositions are provided "per se" thereby.